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By Sara Specht 
It looks like a chunk of wood.
It looks like a small piece of a big tree, something you might grab two-handed to toss into the campfire. Pick it up, and it feels like florist’s foam. It’s light as a dry sponge and flakes away under the weight of your fingers.
What it is is the skeleton of a tree branch: a framework of lignin picked clean of its cellulose mass. The rotted wood is a builder’s nightmare.
It also is a biorefiner’s dream. The sugar compounds that comprise the bulk of all plant tissue, cellulose and hemicellulose, are the building blocks for cellulosic ethanol, a potentially sustainable alternative to fossil-based fuels The challenge is freeing those carbohydrates, unharmed, from the lignin glue that protects it. So far the world has failed to find a way to do it effectively or affordably.
And yet, that perfectly hollowed lignin shell sitting in Jonathan Schilling’s office keeps hope alive that we’ll find a way. Schilling is a professor in the Department of Bioproducts and Biosystems Engineering, and studies the brown rot wood-degrading fungi that made the shell, pests that weaken the wood in decks and buildings.
With a new three-year grant from the U.S. Department of Energy and U.S, Department of Agriculture, Schilling and a team of CFANS researchers hope to discover how these fungi so completely bypass the lignin barrier in plant tissue. They hope to mimic that process to extract the cellulose from wood and other plants for commercial cellulosic ethanol production.
No silver bullet
Found in plant cell walls, cellulose is the most abundant organic compound on Earth, a potentially boundless source of energy. The idea is simple—take a plant, extract the cellulose and ferment it into ethanol or other biofuels—and you have a plentiful source of fuel. But the solution—getting past the lignin that hinders the extraction of cellulose—is complex.
Lignin fills the spaces in a plant cell wall between the carbohydrate components, and it’s the glue gives a plant structure. It also protects the plant from attack from many fungi and bacteria. Lignin itself can be burned for fuel or converted into commercial products, though its potential hasn’t been fully explored. Many paper manufacturers, which break down lignin during the pulping process, burn the lignin byproduct to power the mill.
But until now, scientists have been more interested in getting through it to the cellulose. Cellulosic ethanol processing plants currently use a multistep process that includes a chemical pretreatment to open up or break down the lignin structure, followed by extraction and hydrolysis of the released sugars and fermentation into alcohol for fuel. The process, particularly the pretreatment, is expensive and only partially successful.
“On a good day, getting 80 percent of the cellulose out of the biomass is a success,” says Schilling. “So you’re still leaving 20 grams in 100 on this stubborn plant. There have been a lot of attempts to squeeze out a little extra efficiency, but we’re still not there to commercialization.”
A main focus of research to increase efficiency has been to modify the pretreatment step to neutralize the lignin. Recent studies have concentrated on white rot fungi, which feed on lignin and leave cellulose behind. Others are looking for new chemicals or enzymes that might increase the current system’s effectiveness.
But in the nuisance of brown rot, Schilling saw an extraction process that worked with complete efficiency without damaging a potential value-added lignin component. And brown rot does it without a key enzyme considered critical to the current process. Brown rot performs a pretreatment that allows it to get into a wood’s lignin matrix and sets its own enzymes up for success.
“Many research groups around the world are migrating toward what I call silver bullet enzymes, looking for some enzyme that’s going to perform better than what they already have in hand,” Schilling says. “We’re looking instead at a process. The answer is not that brown rot fungi have a silver bullet enzyme but that they are able to utilize a whole system that allows them complete efficiency.”
In fungi’s footprints
Understanding what brown rot fungi do is easy: step onto an infected log and see it crumble. The fungus saps a tree’s strength long before the mushroom forms. However, understanding exactly how it happens is the challenge.
“There are some key things we need to find out: how does the system function; can we find the mechanism; and can we mimic it to get the cellulose out for biofuels,” says Robert Blanchette, who studies forest pathology and wood degradation in the Department of Plant Pathology.
While Schilling infects wood and plant tissues with three varieties of brown rot to figure out what components are involved in the pretreatment, Blanchette will take a closer look to try to visualize the process and understand the interaction between fungus and biomass on a cellular level. A particular goal is to map out the timing of brown rot’s procedure—the steps and the pieces required have never been defined.
“The odd thing is that lignin seems to trigger this system,” says Blanchette. “You have to have lignin present: the lignin seems to be a triggering mechanism to get everything rolling.”
To test specifically how brown rot’s pretreatment process enhances its own enzymes and affects the different elements of plant tissue, Schilling’s team enlisted the expertise of Timothy Filley at his Purdue Stable Isotope Laboratory. Filley’s diagnostic techniques will use isotopes to identify what and how chemical changes occur during the course of deterioration.
“The idea is looking at wood degradation as a system and giving the fungus some respect,” Schilling says. “Hopefully we can replicate what it does and create a process that mimics nature, in a basic sense.”
Refining the system
Ulrike Tschirner is a lignin chemist in the Department of Bioproducts and Biosystems Engineering. She works with the paper industry, breaking down wood fiber for pulp, but the recent demand for biofuels has brought her onto the project to study how brown rot fungi modifies lignin and the possible applications of that system.
To her, the idea of a biorefinery means using a whole material, not just part of it. And to be economically feasible, she argues, at some point commercial biorefineries will be forced to.
“It’s been really hard to get lignin out of wood without having a byproduct in it,” Tschirner says. “With most pulping you have a sulfur compound, so what you end up with is a material that smells like skunk at the end, which limits its applications. But this is something to get excited about. There are much higher value products that could be made from lignin.”
Right now the bioprocessing pretreatment of cellulosic plant material works to open up the lignin matrix as much as possible, which sacrifices some of the lignin that might be marketed. If the early stages of research are successful and the team is able not only to identify brown rot’s plant degradation mechanism but to mimic it, the changes to current biorefining processes would likely be dramatic. Brown rot seems actually to use fewer steps to bypass lignin than commercial processes. Refineries also tend to keep the costly pretreatment step separate from the later processing.
“In addition to consolidating steps, you also have the potential to save one of your pieces as a value-added product,” says Schilling. “The potential here for a synergy between pretreatment and the enzyme treatments offers a great cost benefit to these companies by doing something that would be truly consolidated bioprocessing.”
Perennial power
Along a road in rural Minnesota, fields stretch into the horizon. Green and maroon and gold sway in a breeze all around the car motoring down the lane. On the left, a neat pattern of lines and curves hypnotizes the eye. To the right is a rolling prairie, flowing in waves to meet the shore of a rich forest.
Imagine you are in that car, and you might be in almost any corner of the state. Now imagine that the power that carries you through that vista is growing all around you.
Economically feasible cellulosic ethanol might be the next step to making this scene a reality: conventional farmers could sell plant byproducts for fuel, and perennial, fast-growing plants might become a high-value agricultural opportunity.
The options presented by cellulosic ethanol seem endless, and Schilling thinks part of the solution might be found by imitating a pest that is just as happy extracting cellulose from alfalfa and corn stover as from spruce and birch. And his grant might be only the first step to driving a plant-powered car.
“Solutions involve multiple approaches,” Schilling says. “And cellulosic ethanol and corn-based ethanol, they’re part of a big blanket. They’re all part of a portfolio of solutions to our problems. That’s the right answer.”
